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The manufacture and properties of lubricating greases
The manufacture greases
and properties
of lubricating
E. F. Jones*
In the first article in this series the properties that a lubricating grease must possess were summarised as: 1 The ability to form a film of lubricant which is not easily removed and which may be required to resist shock loading 2 The ability to form a large mass which will not be easily removed, thus making a seal which will prevent contamination by water, scale and dust 3 The ability to prevent corrosion 4 The ability to resist temperature rises without becoming softened unduly in the process 5 The ability to withstand shear without breakdown of the structure
It was also shown that some properties such as the ability to resist temperature rise and the ability to resist water are largely imparted to the grease by the type of thickener from which the grease is made. The way in which the properties of a grease are affected by the lubricant used as the base was also explained. Other properties, such as stability under shear and oil retention, are largely imparted to the grease by the manufacturing procedure. The remaining properties such as load carrying, resistance to oxidation and in many cases corrosion resistance are functions of the additives present. In this article grease manufacturing procedure and additives will therefore be discussed.
MANUFACTURING
quires heat in order to promote a relatively quick reaction and in many cases water is also added. The process can be summarised as follows:
PROCEDURE
For the sake of convenience the manufacturing procedure for soap based greases can be split into four stages. In the first stage the soap is formed and hence this stage is referred to as saponification. In the second stage the soap is heated in the lubricant until it is dissolved; or as much as possible is dissolved without raising the temperature to a stage where decomposition would occur. The third stage involves cooling the hot mixture, during which time the soap will recrystallize thus imparting its structure to the grease. In the final stage the grease is often homogenized by milling before being de-aerated if necessary and then packaged. In the case of the non-soap greases the thickener is usually dispersed in the lubricant, a wetting agent added and the grease formed by means of homogenization through a mill. Each stage will now be considered separately. Saponification When a mineral acid reacts with an alkali a salt is formed. When a fatty acid reacts with an alkali the salt produced is called a soap.1 Most animal and vegetable fats consist of glycerine in which fatty acids present are combined with glycerine. The number of fatty acid components can be as high as twelve although a few specific fatty acids, such as oleic acid, palmitic acid and stearic acid, usually make up the greater percentage of the fat. A fat is split into fatty acids and glycerine during saponification. This process is usually brought about by reacting the fat with alkali. The fat is first attacked by the alkali to produce fatty acids and glycerine. The fatty acid then reacts with the alkali to produce soap and water. The reaction re* Research Scientist, Berkshire, England.
Esso Research
Centre, Abingdon,
Fat
>
Fatty acid + Alkali The complete
process
Fat + Alkali
&
Fatty acid + Glycerine Soap + Water
is therefore
= Soap + Glycerine
+ Water
For grease-making many different types of fat and fatty acids are used. The cheaper greases, usually made using calcium hydroxide as the alkali, often use the cheaper fats (examples of these are tallows, bone grease and whale oil products). A number of wool grease products are also used. In order to make the more expensive, better quality greases it is quite common to find that specific fats are used in order to impart certain qualities to the grease. For example hydrogenated castor oil products are often used in the production of lithium greases where a high degree of stability is required in the finished grease. Where the purer fats are used it is obviously possible to maintain a much higher standard of product quality and manufacturing control than is possible where mixtures of fats are being used. In the previous article I explained that the main types of alkali usedfor the manufacture of greases are calcium, sodium and lithium. The soap produced in each case imparts different properties to the grease. The consistency of a grease is largely dependent on the amount of soap used to thicken the lubricant. A number 3 NLGI consistency grease may contain as much as 20% soap. The amount of soap necessary in order to produce a grease having a specific consistency is dependent on the lubricant being thickened. For example, a naphthenic mineral oil will in general produce a thicker grease for a given amount of TRIBOLOGY
November
1968
209
soap than a paraffinic mineral oil. During the saponification stage it is common practice to carry out the reaction in a percentage of the total lubricant required and add the remainder of the lubricant at a later stage. It is usual therefore to charge the kettle or autoclave in which the reaction takes place with the hot fat, add some of the lubricant and then add the alkali either in the form of a slurry in oil or as an aqueous solution according to the type of grease being made. Figure 1 shows ingredients being weighed in to a hopper prior to being added to the kettle. Figure 2 shows alkali . being charged to a kettle in the form of an oil slurry. Heating is then commenced at a slow rate with stirring until the saponification reaction is complete. Forming a solution Having formed a soap, heating is continued until the soap is dissolved in the lubricant. It will be recalled that during saponification water and often glycerine are produced. During this stage dehydration may also take place. Although some glycerine may be lost during dehydration this ingredient is mainly retained in the kettle. It acts as a structure modifier in the finished grease and will impart to the grease some measure of heat stability. In order to produce a satisfactory solution, heating may have to be continued until temperatures as high as 250°C are reached. In certain cases already.prepared soaps are used for thickening lubricants. Apart from aluminium greases another example is esters thickened by soaps. If a reaction involving an alkali was carried out in the presence of esters then the esters would be attacked. In these cases the saponification stage is obviously not necessary and the stage of forming a solution with the soap by heating is the first stage after producing an adequate dispersion of the thickener in the lubricant.
Soap recrystallization Having obtained the soap in a solution the next stage is cooling. During this stage the structure is imparted to the grease. A grease can be likened to a sponge soaked in lubricant in which the sponge is the thickener. In this case it will be made of soap. In order to make a satisfactory grease the soap must form a suitable structure. If this is not adequate the grease will become softened unduly when subjected to a shearing action or it may bleed oil to such an extent that the grease may soften on storage until it becomes liquified. (A small amount of oil separation on storage, particularly where grease has been scooped out from a container, is however normal and should not be taken as a signal for alarm). Some years ago the structure of grease became the subject of a considerable amount of research. By means of the electron microscope a magnification of 22 000 is possible and various greases were examined. An example of the use of this instrument in grease development can be illustrated by the lithium greases made from hydrogenated castor oil derivatives. In this case the fatty acid which reacts with the alkali (lithium hydroxide) is known chemically as 12 hydroxystearic acid and consequently these greases are often referred to as lithium hydroxystearate greases. When the lithium hydroxystearate soap dissolved in hot oil is cooled the soap recrystallizes in various forms according to the temperature. Before reaching ambient temperature the soap will therefore have recrystallized in various forms. Some of these forms take the shape of thick fibres and others short thin fibres when viewed under the electron microscope. By controlling the cooling rate various greases can be produced with various structures. The greases produced are then tested by physical tests and also for use in bearings by
Fig 1 A blending hand at a grease plant weighs raw materials before charging grease kettles through pressurized lines. Each material is fed separately to the weigh tank connected to the dial scale (centre) to obtain the correct proportions
Fig 2 Alkali in the form of an oil slurry is fed to a grease kettle. The telescopic device is used to feed a mixture of oils and fats into the kettle which is then heated until saponification is complete
210
TRIBOLOGY
November
1968
Fig 3 An electron micrograph of the structure that gives a good performance
Fig 4 An electron performance
micrograph
of a grease
of a grease that gives a bad
means of rigs. These tests and their significance will be discussed in a later article in this series. Figure 3 shows the structure of a grease giving a good performance as seen under the electron microscope. Figure 4 shows a grease which gave a poor performance. The difference in structure made from the same soap type and base lubricant can be clearly seen. Recent work has however shown that some doubt exists as to the use of this method for examining the structure of grease but, historically the electron microscope has played an important part in advancing grease technology. As was explained, the structure of the grease will depend on the nature of the soap and also the nature of the lubricant being thickened. Whereas lithium greases made from hydrogenated castor oil derivatives and mineral oil may have to be cooled slowly at certain stages in order to produce a desired structure, lithium greases made from esters usually have to be cooled rapidly in order to avoid certain undesir-. able structures being produced. The best characteristics can only be obtained form a grease after a very careful consideration of the ingredients involved and after the most desirable manufacturing conditions have been established. Another aspect of grease manufacture can be illustrated by
greases to be produced. In other cases the yield of grease for a given amount of soap is increased by promoting better dispersion of the soap. After milling it is often necessary to de-aerate the grease and then it is usual to pass the grease through a filter before it is finally packaged.
MANUFACTURE
OF NON-SOAP
GREASE
The description of grease manufacture so far has been concerned with soap thickened products, The preparation of non-soap greases is often a relatively simple process. The thickener is gradually added to the lubricant and is dispersed by an efficient means of stirring. In the case of the organophilic clay ‘bentonite’, for example, it is then ‘wettedby means of a wetting agent which is usually a polar chemical such as acetone. This enables the lubricant which is being thickened to adhere to the surface of the clay. Finally the product is milled in order to bring about stability. Variations of this procedure occur and the above remarks can only be taken as an ouline. Figure 5 shows a bentonite grease being produced in a laboratory mill.
MANUFACTURING
Fig 6 A laboratory-type
EQUIPMENT
Although saponification can be carried out in certain instances at temperatures as low as 60°C it is more usual to require higher temperatures. In many cases the solution stage directly follows the saponification stage and temperatures may therefore reach 250°C during the first two stages of manufacture. A high temperature range is therefore necessary in the reaction vessel. A small amount of water is often added with the alkali in order to assist saponification and if an autoclave is being used, because of the heat involved, pressures up to 100 lb/in2 may be encountered. The equipment used must therefore be capable of withstanding such temperatures and pressures. In many instances it is customary to transfer the contents of the reaction vessels to an open kettle for the cooling and structure forming stage. One autoclave may be used to feed two open kettles. In other circumstances the complete process may be carried out in the open kettle. Efficient heating and cooling media are therefore necessary. Various ancilliary equipment is also available for providing rapid heating or cooling of the kettle contents. The kettles or autoclaves are usually about 8 tons in capacity but obviously the size will depend on the type of processing which is being carried out.
kettle
A suitable means of mixing the kettle contents is also essential in order to avoid overheating of the contents at the heating surfaces. Contra-rotating agitators with scraper blades are often used. Figure 6 shows a laboratory type kettle which is typical of a plant kettle. Milling is usually carried out by passing the grease through a gap between two surfaces. Many types of mill are available. One of the surfaces is normally static and the other rotating. The gap can be accurately controlled and is usually capable of giving a clearance down to a few thousandths of an inch. The speed of the mills varies considerably according to the type being used. Other factors affecting the grease during the milling stage will be the rate at which the grease goes through the mill, the clearance between the rotor and the stator, and the temperature of the grease. Figure 7 shows a laboratory type mill opened up to expose the milling surfaces.
Fig 7 A laboraory-type surfaces 212
TRIROLGGY
mill opened to expose the milling
November
1968
The final stages in grease manufacture involve de-aeration, if necessary, filtering of the grease and finally filling packages. Filtering is carried out in order to remove any impurities which may have been incorporated during the addition of the ingredients or large particles of undispersed soap. As these stages seldom affect the nature of the product they will not be discussed in any detail. Figure 8 shows packages being filled with grease.
heavy loads are applied. Greases containing such additives are usually referred to as EP greases. The additives used contain an active ingredient such as mlphur, chlorine or phosphorus. When heavy loads are applied the active ingredient reacts with the metal surface and produces a film which will prevent welding taking place. The most common load-carrying additives in use are sulphurized fatty materials and lead soaps. Various other compounds are used however according to the type of thickener and lubricant used in the manufacture of the grease. Improvement in corrosion protection Many grease products exhibit a natural protection against corrosion when applied to metal surfaces, particularly those containing wool grease products which are often used as temporary protectives. However other greases without additives do not possess this ability to such a high degree. Corrosion can be brought about by either a grease ingredient (or thermal decomposition of an ingredient) or by an external agent such as salt water if it should break through the grease film. The type of compound used to impart extra protection against corrosion to a grease will therefore depend on several factors. Many compounds are used such as sulphonates, naphthenates, amines and their derivatives, non-ionic surfactants and sometimes inorganic salts such as sodium nitrite.
Fig 8 A filler operating a semi-automatic filling machine at a grease plant. The filling mechanism is manually controlled by valves on the pressurized line and the weight is checked on the scales. Two large hoppers contain the blended greases
PROPERTIES The way structure now been properties 1 The 2 The 3 The
IMPARTED
TO A GREASE BY ADDITIVES
in which greases are made and the most desirable given to a grease in order to impart stability have briefly covered. It will be recalled that three other are necessary. These are: ability to resist shock or heavy loading ability to prevent corrosion ability to remain stable at high temperatures
Although many greases without additives have these properties to some extent, they can be improved by means of additives. The improvements produced can only be illustrated from a knowledge of test procedures and as this will be covered in a subsequent article in this series only a brief mention of the additive will be made. Improvement in load carrying Various additives are incorporated in greases in order to prevent seizure taking place between metal surfaces when
Improvement in thermal stability When a grease is heated for long periods oxidation can be observed to have taken place for example by discolouration or odour. Oxidation does however take place even at ambient temperature. It is therefore desirable to include in the grease an additive which will protect the grease from oxidation. In the case of grease all the ingredients used have to be protected and not only the base lubricant. In selecting the most suitable anti-oxidant many factors therefore have to be taken into account; for example whether the finished grease is slightly acidic or alkaline. A whole variety of compounds have been found satisfactory for use as grease anti-oxidants. Among these the most common are certain amines, certain types of phenols, some sulphur-containing compounds and some organic phosphates. The incorporation of solid lubricants Before concluding the part which additives play in lubricating greases, mention must be made of solid lubricants. The particular properties of laminar solids as lubricants, such as high resistance to thermal breakdown and the ability to adhere to.metal surfaces, are well known. In certain cases these products are incorporated into greases to impart extra anti-seize properties to the grease. Examples of two such compounds are graphite and molybdenum disulphide. In the first :wo articles in this series an effort has been made to explain the nature of lubricating grease and the properties that these products must have in order to fulfil their functions in the overall pattern of lubrication. The way in which these properties are imparted by means of the ingredients used in their composition, the carefully controlled manufacturing procedure that often has to be used and the improvement in certain properties which can be obtained by means of additives has also been covered. It should however be appreciated that in these articles the subject has only been dealt with in general terms and that it has not been possible to cover the many specialities which exist amongst the vast range of lubricating greases. REFERENCES 1
E. F. Jones ‘Lubrication grease’, Vol 1,No 3 (1968) p 161
TRIBOLOGY
Tribology
November
1968
213
and properties
of lubricating
E. F. Jones*
In the first article in this series the properties that a lubricating grease must possess were summarised as: 1 The ability to form a film of lubricant which is not easily removed and which may be required to resist shock loading 2 The ability to form a large mass which will not be easily removed, thus making a seal which will prevent contamination by water, scale and dust 3 The ability to prevent corrosion 4 The ability to resist temperature rises without becoming softened unduly in the process 5 The ability to withstand shear without breakdown of the structure
It was also shown that some properties such as the ability to resist temperature rise and the ability to resist water are largely imparted to the grease by the type of thickener from which the grease is made. The way in which the properties of a grease are affected by the lubricant used as the base was also explained. Other properties, such as stability under shear and oil retention, are largely imparted to the grease by the manufacturing procedure. The remaining properties such as load carrying, resistance to oxidation and in many cases corrosion resistance are functions of the additives present. In this article grease manufacturing procedure and additives will therefore be discussed.
MANUFACTURING
quires heat in order to promote a relatively quick reaction and in many cases water is also added. The process can be summarised as follows:
PROCEDURE
For the sake of convenience the manufacturing procedure for soap based greases can be split into four stages. In the first stage the soap is formed and hence this stage is referred to as saponification. In the second stage the soap is heated in the lubricant until it is dissolved; or as much as possible is dissolved without raising the temperature to a stage where decomposition would occur. The third stage involves cooling the hot mixture, during which time the soap will recrystallize thus imparting its structure to the grease. In the final stage the grease is often homogenized by milling before being de-aerated if necessary and then packaged. In the case of the non-soap greases the thickener is usually dispersed in the lubricant, a wetting agent added and the grease formed by means of homogenization through a mill. Each stage will now be considered separately. Saponification When a mineral acid reacts with an alkali a salt is formed. When a fatty acid reacts with an alkali the salt produced is called a soap.1 Most animal and vegetable fats consist of glycerine in which fatty acids present are combined with glycerine. The number of fatty acid components can be as high as twelve although a few specific fatty acids, such as oleic acid, palmitic acid and stearic acid, usually make up the greater percentage of the fat. A fat is split into fatty acids and glycerine during saponification. This process is usually brought about by reacting the fat with alkali. The fat is first attacked by the alkali to produce fatty acids and glycerine. The fatty acid then reacts with the alkali to produce soap and water. The reaction re* Research Scientist, Berkshire, England.
Esso Research
Centre, Abingdon,
Fat
>
Fatty acid + Alkali The complete
process
Fat + Alkali
&
Fatty acid + Glycerine Soap + Water
is therefore
= Soap + Glycerine
+ Water
For grease-making many different types of fat and fatty acids are used. The cheaper greases, usually made using calcium hydroxide as the alkali, often use the cheaper fats (examples of these are tallows, bone grease and whale oil products). A number of wool grease products are also used. In order to make the more expensive, better quality greases it is quite common to find that specific fats are used in order to impart certain qualities to the grease. For example hydrogenated castor oil products are often used in the production of lithium greases where a high degree of stability is required in the finished grease. Where the purer fats are used it is obviously possible to maintain a much higher standard of product quality and manufacturing control than is possible where mixtures of fats are being used. In the previous article I explained that the main types of alkali usedfor the manufacture of greases are calcium, sodium and lithium. The soap produced in each case imparts different properties to the grease. The consistency of a grease is largely dependent on the amount of soap used to thicken the lubricant. A number 3 NLGI consistency grease may contain as much as 20% soap. The amount of soap necessary in order to produce a grease having a specific consistency is dependent on the lubricant being thickened. For example, a naphthenic mineral oil will in general produce a thicker grease for a given amount of TRIBOLOGY
November
1968
209
soap than a paraffinic mineral oil. During the saponification stage it is common practice to carry out the reaction in a percentage of the total lubricant required and add the remainder of the lubricant at a later stage. It is usual therefore to charge the kettle or autoclave in which the reaction takes place with the hot fat, add some of the lubricant and then add the alkali either in the form of a slurry in oil or as an aqueous solution according to the type of grease being made. Figure 1 shows ingredients being weighed in to a hopper prior to being added to the kettle. Figure 2 shows alkali . being charged to a kettle in the form of an oil slurry. Heating is then commenced at a slow rate with stirring until the saponification reaction is complete. Forming a solution Having formed a soap, heating is continued until the soap is dissolved in the lubricant. It will be recalled that during saponification water and often glycerine are produced. During this stage dehydration may also take place. Although some glycerine may be lost during dehydration this ingredient is mainly retained in the kettle. It acts as a structure modifier in the finished grease and will impart to the grease some measure of heat stability. In order to produce a satisfactory solution, heating may have to be continued until temperatures as high as 250°C are reached. In certain cases already.prepared soaps are used for thickening lubricants. Apart from aluminium greases another example is esters thickened by soaps. If a reaction involving an alkali was carried out in the presence of esters then the esters would be attacked. In these cases the saponification stage is obviously not necessary and the stage of forming a solution with the soap by heating is the first stage after producing an adequate dispersion of the thickener in the lubricant.
Soap recrystallization Having obtained the soap in a solution the next stage is cooling. During this stage the structure is imparted to the grease. A grease can be likened to a sponge soaked in lubricant in which the sponge is the thickener. In this case it will be made of soap. In order to make a satisfactory grease the soap must form a suitable structure. If this is not adequate the grease will become softened unduly when subjected to a shearing action or it may bleed oil to such an extent that the grease may soften on storage until it becomes liquified. (A small amount of oil separation on storage, particularly where grease has been scooped out from a container, is however normal and should not be taken as a signal for alarm). Some years ago the structure of grease became the subject of a considerable amount of research. By means of the electron microscope a magnification of 22 000 is possible and various greases were examined. An example of the use of this instrument in grease development can be illustrated by the lithium greases made from hydrogenated castor oil derivatives. In this case the fatty acid which reacts with the alkali (lithium hydroxide) is known chemically as 12 hydroxystearic acid and consequently these greases are often referred to as lithium hydroxystearate greases. When the lithium hydroxystearate soap dissolved in hot oil is cooled the soap recrystallizes in various forms according to the temperature. Before reaching ambient temperature the soap will therefore have recrystallized in various forms. Some of these forms take the shape of thick fibres and others short thin fibres when viewed under the electron microscope. By controlling the cooling rate various greases can be produced with various structures. The greases produced are then tested by physical tests and also for use in bearings by
Fig 1 A blending hand at a grease plant weighs raw materials before charging grease kettles through pressurized lines. Each material is fed separately to the weigh tank connected to the dial scale (centre) to obtain the correct proportions
Fig 2 Alkali in the form of an oil slurry is fed to a grease kettle. The telescopic device is used to feed a mixture of oils and fats into the kettle which is then heated until saponification is complete
210
TRIBOLOGY
November
1968
Fig 3 An electron micrograph of the structure that gives a good performance
Fig 4 An electron performance
micrograph
of a grease
of a grease that gives a bad
means of rigs. These tests and their significance will be discussed in a later article in this series. Figure 3 shows the structure of a grease giving a good performance as seen under the electron microscope. Figure 4 shows a grease which gave a poor performance. The difference in structure made from the same soap type and base lubricant can be clearly seen. Recent work has however shown that some doubt exists as to the use of this method for examining the structure of grease but, historically the electron microscope has played an important part in advancing grease technology. As was explained, the structure of the grease will depend on the nature of the soap and also the nature of the lubricant being thickened. Whereas lithium greases made from hydrogenated castor oil derivatives and mineral oil may have to be cooled slowly at certain stages in order to produce a desired structure, lithium greases made from esters usually have to be cooled rapidly in order to avoid certain undesir-. able structures being produced. The best characteristics can only be obtained form a grease after a very careful consideration of the ingredients involved and after the most desirable manufacturing conditions have been established. Another aspect of grease manufacture can be illustrated by
greases to be produced. In other cases the yield of grease for a given amount of soap is increased by promoting better dispersion of the soap. After milling it is often necessary to de-aerate the grease and then it is usual to pass the grease through a filter before it is finally packaged.
MANUFACTURE
OF NON-SOAP
GREASE
The description of grease manufacture so far has been concerned with soap thickened products, The preparation of non-soap greases is often a relatively simple process. The thickener is gradually added to the lubricant and is dispersed by an efficient means of stirring. In the case of the organophilic clay ‘bentonite’, for example, it is then ‘wettedby means of a wetting agent which is usually a polar chemical such as acetone. This enables the lubricant which is being thickened to adhere to the surface of the clay. Finally the product is milled in order to bring about stability. Variations of this procedure occur and the above remarks can only be taken as an ouline. Figure 5 shows a bentonite grease being produced in a laboratory mill.
MANUFACTURING
Fig 6 A laboratory-type
EQUIPMENT
Although saponification can be carried out in certain instances at temperatures as low as 60°C it is more usual to require higher temperatures. In many cases the solution stage directly follows the saponification stage and temperatures may therefore reach 250°C during the first two stages of manufacture. A high temperature range is therefore necessary in the reaction vessel. A small amount of water is often added with the alkali in order to assist saponification and if an autoclave is being used, because of the heat involved, pressures up to 100 lb/in2 may be encountered. The equipment used must therefore be capable of withstanding such temperatures and pressures. In many instances it is customary to transfer the contents of the reaction vessels to an open kettle for the cooling and structure forming stage. One autoclave may be used to feed two open kettles. In other circumstances the complete process may be carried out in the open kettle. Efficient heating and cooling media are therefore necessary. Various ancilliary equipment is also available for providing rapid heating or cooling of the kettle contents. The kettles or autoclaves are usually about 8 tons in capacity but obviously the size will depend on the type of processing which is being carried out.
kettle
A suitable means of mixing the kettle contents is also essential in order to avoid overheating of the contents at the heating surfaces. Contra-rotating agitators with scraper blades are often used. Figure 6 shows a laboratory type kettle which is typical of a plant kettle. Milling is usually carried out by passing the grease through a gap between two surfaces. Many types of mill are available. One of the surfaces is normally static and the other rotating. The gap can be accurately controlled and is usually capable of giving a clearance down to a few thousandths of an inch. The speed of the mills varies considerably according to the type being used. Other factors affecting the grease during the milling stage will be the rate at which the grease goes through the mill, the clearance between the rotor and the stator, and the temperature of the grease. Figure 7 shows a laboratory type mill opened up to expose the milling surfaces.
Fig 7 A laboraory-type surfaces 212
TRIROLGGY
mill opened to expose the milling
November
1968
The final stages in grease manufacture involve de-aeration, if necessary, filtering of the grease and finally filling packages. Filtering is carried out in order to remove any impurities which may have been incorporated during the addition of the ingredients or large particles of undispersed soap. As these stages seldom affect the nature of the product they will not be discussed in any detail. Figure 8 shows packages being filled with grease.
heavy loads are applied. Greases containing such additives are usually referred to as EP greases. The additives used contain an active ingredient such as mlphur, chlorine or phosphorus. When heavy loads are applied the active ingredient reacts with the metal surface and produces a film which will prevent welding taking place. The most common load-carrying additives in use are sulphurized fatty materials and lead soaps. Various other compounds are used however according to the type of thickener and lubricant used in the manufacture of the grease. Improvement in corrosion protection Many grease products exhibit a natural protection against corrosion when applied to metal surfaces, particularly those containing wool grease products which are often used as temporary protectives. However other greases without additives do not possess this ability to such a high degree. Corrosion can be brought about by either a grease ingredient (or thermal decomposition of an ingredient) or by an external agent such as salt water if it should break through the grease film. The type of compound used to impart extra protection against corrosion to a grease will therefore depend on several factors. Many compounds are used such as sulphonates, naphthenates, amines and their derivatives, non-ionic surfactants and sometimes inorganic salts such as sodium nitrite.
Fig 8 A filler operating a semi-automatic filling machine at a grease plant. The filling mechanism is manually controlled by valves on the pressurized line and the weight is checked on the scales. Two large hoppers contain the blended greases
PROPERTIES The way structure now been properties 1 The 2 The 3 The
IMPARTED
TO A GREASE BY ADDITIVES
in which greases are made and the most desirable given to a grease in order to impart stability have briefly covered. It will be recalled that three other are necessary. These are: ability to resist shock or heavy loading ability to prevent corrosion ability to remain stable at high temperatures
Although many greases without additives have these properties to some extent, they can be improved by means of additives. The improvements produced can only be illustrated from a knowledge of test procedures and as this will be covered in a subsequent article in this series only a brief mention of the additive will be made. Improvement in load carrying Various additives are incorporated in greases in order to prevent seizure taking place between metal surfaces when
Improvement in thermal stability When a grease is heated for long periods oxidation can be observed to have taken place for example by discolouration or odour. Oxidation does however take place even at ambient temperature. It is therefore desirable to include in the grease an additive which will protect the grease from oxidation. In the case of grease all the ingredients used have to be protected and not only the base lubricant. In selecting the most suitable anti-oxidant many factors therefore have to be taken into account; for example whether the finished grease is slightly acidic or alkaline. A whole variety of compounds have been found satisfactory for use as grease anti-oxidants. Among these the most common are certain amines, certain types of phenols, some sulphur-containing compounds and some organic phosphates. The incorporation of solid lubricants Before concluding the part which additives play in lubricating greases, mention must be made of solid lubricants. The particular properties of laminar solids as lubricants, such as high resistance to thermal breakdown and the ability to adhere to.metal surfaces, are well known. In certain cases these products are incorporated into greases to impart extra anti-seize properties to the grease. Examples of two such compounds are graphite and molybdenum disulphide. In the first :wo articles in this series an effort has been made to explain the nature of lubricating grease and the properties that these products must have in order to fulfil their functions in the overall pattern of lubrication. The way in which these properties are imparted by means of the ingredients used in their composition, the carefully controlled manufacturing procedure that often has to be used and the improvement in certain properties which can be obtained by means of additives has also been covered. It should however be appreciated that in these articles the subject has only been dealt with in general terms and that it has not been possible to cover the many specialities which exist amongst the vast range of lubricating greases. REFERENCES 1
E. F. Jones ‘Lubrication grease’, Vol 1,No 3 (1968) p 161
TRIBOLOGY
Tribology
November
1968
213